Mechanical Properties of Fluids

10 previous-year questions from this chapter every option and the correct answer, free · where the marks are

1. Pressure, Pascal's law, buoyancy

Exam focus: Buoyant force depends only on the volume of fluid displaced and the fluid's density — once an object is fully submerged, pushing it deeper doesn't change the buoyant force, since the displaced volume stays the same.

Hydrostatics: Archimedes' Principle, Buoyancy and Floatation

Physics Wallah - Alakh Pandey

Covers pressure/hydrostatics, Archimedes' principle, buoyancy and floatation; Pascal's law is part of the same hydrostatics treatment though not named in the title.

2. Viscosity, Stokes' law, terminal velocity

Exam focus: Terminal velocity under Stokes' law scales as the square of the radius, not linearly — doubling a drop's radius quadruples its terminal speed, and NEET tests this exponent directly more often than the formula's derivation.

Stokes' Law and Terminal Velocity

Physics Wallah - Alakh Pandey

Directly covers Stokes' law and terminal velocity; viscosity itself is introduced briefly here, having been covered more fully in the immediately preceding video of the same series.

3. Bernoulli's principle and its applications

Exam focus: Bernoulli's equation assumes non-viscous, incompressible, streamline flow — the familiar 'pressure drops where speed increases' rule also silently assumes the same height, so applying it to turbulent or viscous flow is already outside its own assumptions.

Bernoulli's Principle, Venturimeter and Equation of Continuity

Physics Wallah - Alakh Pandey

Covers Bernoulli's principle and the venturimeter application directly; aerofoil lift gets only a brief standard mention.

4. Surface tension, capillarity, excess pressure

Exam focus: Excess pressure is 4T/r inside a soap bubble but only 2T/r inside a plain liquid drop or a submerged air bubble — the extra factor comes from the soap film having two free surfaces instead of one.

Surface Tension: Introduction and Surface Energy

Physics Wallah - Alakh Pandey

Introduces surface tension and surface energy thoroughly; this is part 1 of a 4-part series — capillarity and excess pressure in drops/bubbles are covered in the later videos of the same series, not this one.

PhysicsNEET 2025show ▾

Consider a water tank shown in the figure. It has one wall at

x = L

and can be taken to be very wide in the

z

direction. When filled with a liquid of surface tension

S

and density

ρ

, the liquid surface makes an angle

θ_{0}

(

θ_{0} ≪ 1

) with the

x

-axis at

x = L

. If

y (x)

is the height of the surface, then the equation for

y (x)

is (take

sin θ (x) \approx tan θ (x) \approx \frac{d y}{d x}

;

g

is the acceleration due to gravity): Question 1 as printed in the NEET 2025 booklet

A. $\frac{d ^{2} y}{d x ^{2}} = \frac{ρ g}{S} x$
B. $\frac{d ^{2} y}{d x ^{2}} = \frac{ρ g}{S} y$ ✓
C. $\frac{d ^{2} y}{d x ^{2}} = \frac{ρ g}{S}$
D. $\frac{d y}{d x} = \frac{ρ g}{S} x$

Solution

At height $y$ the hydrostatic pressure $ρ g y$ is balanced by the surface-tension pressure $S \times$ (curvature). For a nearly flat surface the curvature is $\frac{d ^{2} y}{d x ^{2}}$ , so $S \frac{d ^{2} y}{d x ^{2}} = ρ g y$ , i.e. $\frac{d ^{2} y}{d x ^{2}} = \frac{ρ g}{S} y$ .

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PhysicsNEET 2024show ▾

A thin flat circular disc of radius 4.5 cm is placed gently over the surface of water. If the surface tension of water is

0.07 N m^{- 1}

, then the excess force required to take it away from the surface is:

A.198 N
B.1.98 mN
C.99 N
D.19.8 mN✓

Solution

A disc lifted off a liquid surface is held by surface tension along its full circumference: $F = T \times 2 π r = 0.07 \times 2 π \times 0.045 \approx 1.98 \times 1 0^{- 2}$ N $= 19.8$ mN.

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PhysicsNEET 2023show ▾

The amount of energy required to form a soap bubble of radius 2 cm from a soap solution is nearly (surface tension of soap solution

= 0.03 N m^{- 1}

A. $30.16 \times 1 0^{- 4}$ J
B. $5.06 \times 1 0^{- 4}$ J
C. $3.01 \times 1 0^{- 4}$ J✓
D. $50.1 \times 1 0^{- 4}$ J

The worked solution is part of a paid pack.

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PhysicsNEET 2022show ▾

If a soap bubble expands, the pressure inside the bubble:

A.Is equal to the atmospheric pressure
B.Decreases✓
C.Increases
D.Remains the same

The worked solution is part of a paid pack.

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PhysicsNEET 2020show ▾

A capillary tube of radius

r

is immersed in water and water rises in it to a height

h

. The mass of the water in the capillary is 5 g. Another capillary tube of radius

2 r

is immersed in water. The mass of water that will rise in this tube is:

A.2.5 g
B.5.0 g
C.10.0 g✓
D.20.0 g

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PhysicsNEET 2019show ▾

A soap bubble, having a radius of 1 mm, is blown from a detergent solution having a surface tension of

2.5 \times 1 0^{- 2} N/m

. The excess pressure inside the bubble equals the pressure at a point

Z_{0}

below the free surface of water in a container. Taking

g = 10 m/ s^{2}

and density of water

= 1 0^{3} kg/ m^{3}

, the value of

Z_{0}

is:

A.100 cm
B.10 cm
C.1 cm✓
D.0.5 cm

The worked solution is part of a paid pack.

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1. Pressure, Pascal's law, buoyancy

Pressure, Pascal's law and buoyancy

2. Viscosity, Stokes' law, terminal velocity

Viscosity, Stokes' law and terminal velocity

3. Bernoulli's principle and its applications

Bernoulli's principle and its applications

4. Surface tension, capillarity, excess pressure

Surface tension, capillarity and excess pressure